Glutamine, a nonessential amino acid, is preferred fuel for rapidly proliferating cells in human body. Those cells include the enterocytes in small intestine, lymphocytes, macrophages, and fibroblasts. Glutamine also transports nitrogen between tissues and serves as a precursor to glutathione which is a potent antioxidant. A healthy human body contains abundant glutamine, either from diet or from skeletal muscle tissue that synthesizes it.